1. Field of the Invention
The present invention relates to an image signal decoding apparatus for decoding digital image signals.
2. Related Background Art
As a coding method, there is known a DC separation vector quantization method by which image data are efficiently coded on a block unit (block-by-block) basis. According to this method, an image represented by image data is divided into a plurality of blocks, and the distribution of levels of image data within a block concentrates near at the 0 level of the image data has subtracted from each level the mean value of the image data within the block. By positively using this fact, the image data with the mean value subtracted therefrom are vector-quantized, and the quantized data and mean value data are transmitted.
During the transmission of image data coded in accordance with the above-described coding method, if the information representative of the characteristic of a block, such as the mean value data of image data within the block, becomes deficient, it becomes impossible to decode the coded image data into the original image data on the block unit basis. The image reproduced by using such deficient decoded image data becomes visually poor. It is therefore necessary to reduce the visual deterioration by performing, on the block unit basis, an interpolation of the image data of a block without the mean value data, with image data of another block.
If an inter-frame interpolation, which uses image data of an image already transmitted, is used for an interpolation of image data of a block without the mean value data, with image data of another block on the block unit basis, the image at a still image area can be improved visually to a large extent. In order to decide if a block to be interpolated belongs to a still image area, the motion amount of an image represented by image data in a block capable of being decoded and near the first-mentioned block, is estimated. If it is judged that the block incapable of being decoded belongs to a still image area, the block incapable of being decoded is interpolated by means of the inter-frame interpolation. If a nearby block capable of being decoded is used in detecting motion of an image represented by image data in a block, the image motion detection cannot be obtained until the nearby block capable of being decoded has been completely decoded. An immediate interpolation at that time is therefore not possible, leaving a disadvantage in decoding. Further, all the image data are required to be compared for the purpose of interpolation upon occurrence of a mean value data deficiency, so that it is wasteful in that a long calculation time and a large number of calculation circuits become necessary.